Tolerance of Gentianella campestris in relation to damage intensity: an interplay between apical dominance and herbivory

Meristem allocation models suggest that the patterns of compensatory regrow th responses following grazing vary, depending on (i) the number of latent meristems that escape from being damaged, and (ii) the activation sensitivi ty of the meristems in relation to the degree of damage. We examined the sh ape of compensatory responses in two late-flowering populations (59 degrees 20'N and 65 degrees 45'N) of the field gentian. Plants of equal initial si zes were randomly assigned to four treatment groups with 0, 10, 50 and 75% removal of the main stalk. The plants were clipped before flowering, and th eir performance was studied at the end of the growing season. The northern population showed a linear decrease in shoot biomass and fecundity with inc reasing biomass removal, while the response in the southern population was quadratic with maximum performance at the damage level of 50% clipping. Thi s nonlinear shape depended upon the activation sensitivity of dormant meris tems in relation to their position along the main stem. The highest plant p erformance was achieved by inflicting intermediate damage which induced reg rowth from basally located meristems. In contrast, the topmost branches too k over the dominance role of the main stem after minor apical damage (10% c lipping). Consequently, the breakage of apical dominance is a necessary pre condition of vigorous regrowth in this species. However, compensation in th e field gentian is unlikely to be a mere incidental by-product of apical do minance. The ability to regrow from basally located meristems that escape f rom being damaged by grazing may well be a sign of adaptation to moderate l evels of shoot damage.